Chapter 5 Light Intensity Dependent Photo-energy Conversion/Storage Efficiency of Dye-sensitized Solar
5.4 Experimental Section
Storage-Electrode Fabrication Storage-electrode was composed of LiMn2O4as an active material, Super P(Timcal) carbon black as a conduction enhancer and polyvinylidene fluoride (PVdF, SOLVAY Solef 5130) as a binder (weight ratio of 8:1:1) on the Au-coated FTO. Firstly, PVdF powder was dissolved in N-methyl pyrrolidinone (NMP). Mixture of LiMn2O4 and Super P was finely dispersed into the PVdF/NMP paste with stirring. The Au-coated FTO glass was pre-drilled for electrolyte injection and masked using kapton tape (25.4 μm). The LiMn2O4slurry was dropped onto the masked substrate, followed by drying at 110 °C for 1 h and naturally cooling to 25 °C. The active area was 0.6×0.6 cm2and the loading density of LiMn2O ranged 1 ~ 2.2 mg/cm2.
Cell Assembly Storage-electrode and LISICON membrane were attached using the 60-μm Surlyn® resin (Meltonix 1170-60, Solaronix SA) at 110 °C. Then, upper side of LISICON was masked with kapton tape and Pt-sputtered for making 3 mm width Pt layer as a charge collector using FEI-sputter (K575X, EMITECH), followed by electrodeposition of poly(3,4-ethylenedioxythiophene) (PEDOT) on that as a catalytic layer. The electrodeposition was carried out applying a constant current (+0.5 mA) for 30 s in an acetonitrile dispersion of EDOT containing 0.01 M EDOT (97%, Aldrich) and 0.1 M LiClO4(99.99% trace metals basis, Aldrich). Then, photo-electrode was attached on that using 2 layers of 60-μm Surlyn®resin at 110 °C. Both internal spaces for photo-electrode side and storage-electrode side were filled with catholyte and 0.8 M LiClO4in acetonitrile, respectively. I─/I3─catholyte was composed of 0.2 M I2(≥ 99.8%, Aldrich), 0.1 M LiI (99.9% trace metals basis, Aldrich), 0.05 M guanidine thiocyanate (≥ 97%, Aldrich), 0.6 M 1,2- dimethyl-3-propylimidazolium iodide (DMPII, Solaronix SA), and 0.5 M 4-tert-butylpridine (96%, Aldrich) in acetonitrile. Co2+/3+(bpy)3and Cu+/2+(dmp)2catholytes were composed of 0.25 M Co2+(bpy)3(PF6)2, 0.05 M Co3+(bpy)3(PF6)3, 0.1 M LiClO4, and 0.5 M 4-tert-butylpyridine in acetonitrile and 0.2 M Cu+(dmp)2TFSI, 0.04 M Cu2+(dmp)2TFSI/Cl, 0.1 M LiClO4, and 0.5 M 4-tert-butylpyridine in acetonitrile, respectively.
Characterization The absorbance spectra of dye desorption solution were recorded by the R928 photomultiplier tube of a UV-vis-NIR spectrophotometer (Cary 5000, Agilent Technologies, Inc.). The surface morphology of LiMn2O4film was observed by field emission-scanning electron micro-scope (FE- SEM, Haitachi, S-4800). To identify the crystallinity of the graphitized LiMn2O4, X-ray diffraction (XRD) was carried out in a D8 ADVANCE system equipped with a DAVINCI (Bruker AXS) diffractometer using Cu Kα radiation operated at 40 kV and 10 mA. All electrochemical analyses were performed on potentiostats/galvanostats (BioLogic VMP3).
Photo-Charging/Galvanostatic Discharging Photo-charging current was measured using chronoamperometry technique applying 0 V of dc bias under the illumination. Following discharging was carried out using chronopotentiometry technique with various constant currents. Standard one sun condition (AM 1.5, 100 mW cm−2) was simulated by photovoltaic efficiency measurement system (IQE-200, Newport Corporation). A commercial light-emitting diode lighting power (LG Innotek Co., Ltd.) was employed to simulate the indoor light intensity.
Cyclic Voltammetry Cyclic voltammetry was carried out at 100 mV s−1of scan rate in a 3-electrode system containing Ag/AgCl (in a saturated aqueous potassium chloride solution) as the reference electrode and Pt wire as the counter-electrode in supporting electrolyte (0.1 M LiClO4in acetonitrile). Standard redox potentials were referenced versus that of ferrocene/ferrocenium (+0.63 V vs.NHE).
Controlled Intensity Modulated Photo-spectroscopy To study the photo-current and photo-voltage efficiency frequency response of DSSBs, controlled intensity modulated photo-spectroscopy (CIMPS) technique was carried out on a photoelectrochemical workstation (ZENNIUM XPOT, ZAHNER-elektrik GmbH & Co. KG) equipped with a frequency response analyzer and an automatically intensity-controlled light-emitting diode (503 nm). A small-amplitude sinusoidal pulse (~10% of dc potential) was applied to light source bias with frequency sweep from 105to 10−1Hz.
Linear Sweep Voltammetry and Impedance Spectroscopic Analyses Linear sweep voltammetry was carried out at 100 mV s−1 of scan rate with the symmetric cells consisting of two PEDOT-coated FTO substrates and the electrolyte same with the catholyte in DSSB. Electrochemical impedance spectroscopy was carried out with the same symmetric cells, applying a 10 mV sinusoidal pulse to a dc bias equivalent to the open circuit voltage (0 V) with the frequency sweep from 106to 10−1Hz. All obtained Nyquist plots were fitted with suitable equivalent circuits using ZView software (Scribner Associates, Inc.).
Time-Correlated Single Photon Counting Transient photoluminescence lifetime was measured on fluorescence lifetime spectrometer (FluoTime 300, PicoQuant GmbH), operated in time-correlated single
The evaluation of electrochemical kinetic parameters For the measurements, I prepared symmetric cell that consists of two PEDOT-coated FTO electrodes and the electrolyte as depicted in Figure 5.13a.
The rate constant (k0) for charge transfer near electrode surface and ionic diffusion coefficient (D) values are summarized in Table 5.1. For both factors, symmetric cells containing I─/I3─ electrolyte always give the highest values, indicative of the fastest reaction and diffusion of I─/I3─. The impedance spectroscopy analysis strongly supports linear sweep voltammetry results in terms of electrode/electrolyte interface impedance and Nernst diffusion impedance.
Figure 5.13cand 5.13dpresent Nyquist plots obtained from symmetric cells with different electrolyte and equivalent circuit we adopted for fitting, respectively. It is apparent that I─/I3─and Co2+/3+(bpy)3feature two semicircles in impedance spectra, while Cu+/2+(dmp)2 is characterized by three semicircles. Nyquist plots obtained from the symmetric cells are generally composed of two semicircles, corresponding to charge transfer resistance at the electrode/electrolyte interface (RCT, Ed/El) at high frequency region and Nernst diffusion impedance in the electrolyte (Nbulk) at low frequency region, respectively. However, when the electrode surface has a porous structure, additional element should be introduced to circuit to consider Nernst diffusion impedance within the electrode pores (Npore).59This element has been known to appear at higher frequency region than that of RCT, Ed/El. Therefore, we attributed additional semicircle at high frequency region (200~2.5 kHz) with Cu+/2+(dmp)2 electrolyte to sluggish ionic diffusion within the of PEDOT pore. For the other cases, diffusion limitation in PEDOT pore is negligible and thus the Nporedo not appear. All RCT, Ed/Eland Nbulkvalues are summarized in Table 5.1. It is noteworthy that I─/I3─provides very low RCT(0.14 Ω) and Nbulk(0.55 Ω).
Figure 5.13(a) The structure of symmetric cell used for (b) linear sweep voltammetry and (c) impedance analyses for rate constant (k0) and ionic diffusion coefficient (D) of redox mediators, respectively. k0and D were estimated using the exchange current (I0) and limiting current (Ilim) as following Eq.5.6 and Eq.5.7:
= (5.6)
= 2 ⁄ (5.7) where nis the number of electrons transferred in the electrochemical reaction, Fis the faraday constant, A is the active area, CO*and CR*are bulk concentrations of redox species, αis the transfer coefficient (≈ 0.5),
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Acknowledgement
2013 년 12 월 끝자락, 권태혁 교수님과의 면담을 위해 처음 UNIST 를 방문했던 기억이 생생합니다. 그렇게 2014 년부터 시작된 저의 대학원 생활이 어느새 막바지에 이르렀습니다.
짧고도 길었던 지난 4 년 6 개월간의 시간은 저의 인생에 큰 영향을 주었고 돌이켜 보면 많은 분들의 격려와 도움 덕분에 무사히 학위 과정을 마칠 수 있었던 것 같습니다. 먼저 대학원 생활동안 저에게 끊임없는 격려와 가르침 주신 권태혁 교수님께 진심으로 감사의 인사를 드립니다. 교수님으로부터 받은 학문적 소양과 따뜻함은 앞으로 사회 생활을 헤쳐 나감에 있어 구심점이 될 것입니다. 부족한 저를 여기까지 이끌어 주셔서 감사합니다. 항상 초심을 잃지 않고 정진하도록 하겠습니다. 항상 저를 믿고 바라봐 주신 부모님, 많이 사랑합니다. 두 분은 순탄치만은 않았던 대학원 생활을 제가 견디고 달려올 수 있었던 원동력이었습니다. 하나밖에 없는 아들 잘 하고 있으니 조금만 더 참고 기다려주세요.
아울러 우리 ER Lab 울타리 안에 많은 동료들에게도 감사의 인사를 전합니다. Co-worker 이자 실험실 내 유일한 형인 정수형, 도중 연구주제가 바뀌어 힘든 부분이 있겠지만 머지 않아 좋은 결과 있을 거예요. 파이팅 하세요. 실험실 내 유일한 동갑내기인 현규, 그래서 그런지 알게 모르게 마음속으로 의지가 되었던 것 같네 ㅋㅋ 어서 DSSB 로 좋은 결과 만들어 보자. 나도 많이 도울 게. Co-worker 이자 유일한 입학 동기 현오 SHIN, 돌이켜 보면 너한테 여러 측면에서 많이 의지를 했던 것 같네. 고독한 소주파인 나의 술친구가 되어줘서 고마웠음 ㅋ 남은 연구도 잘 진행해서 좋은 결실 이루길 바랄게. 넌 꼭 잘 될 거다 ㅋ 랩장으로써 ERL 을 이끌어 가고 있는 현탁, 연구적인 부분은 두말할 것도 없고 실험실 생활도 잘하고 있는 것 같아 보기 좋다. 형으로써 많은 도움주지 못해서 미안해 ㅜㅜ... 다재 다능한 ERL MC 광민, 몇 안되는 DSC 디바이스 가이인데 많은 도움주지 못해서 미안하네. 항상 정진하는 마음으로 연구 잘 진행해서 좋은 결실을 맺길 바라.
오피스 옆자리에서 묵묵히 논문을 보던 정혁, 벌써 졸업한지 2 년이 되었네. 좋은데 취직했다는 소식 들었어. 멀리서나마 축하의 메시지 보낸다. 막 사회에 나가서 활동하고 있을 언영, 같은 부산 주민이라고 이래저래 놀려 대긴 했지만 너그러운 마음으로 이해하길... 그게 부산 스타일 아니겠나 ㅎ 점점 상남자로 거듭나고 있는 정승, 처음 UNIST 와서 봤던 너의 앳된(?) 이미지가 아직도 생생하다 ㅋㅋ 분야는 다르지만 날이 갈수록 너의 연구들이 성숙하는 것 같아 보기 좋네.